This invention relates to a multiple access communication systems and, in particular, to a communications system utilizing frequency division, code division and time division multiple access techniques for providing efficient use of frequency spectrum while supporting a large number of transmitters sharing a common frequency band by having all transmitters synchronized to a common timing reference.
Frequency division multiple access (FDMA) systems involve assigning each user a specific frequency for its respective transmission. Accordingly, a high number of users within an FDMA system requires a large frequency band. For example, if ten users are desired within an FDMA system, ten separate and independent frequencies would be required.
Time division multiple access (TDMA) systems involve multiple users sharing a common frequency but each user transmits at a specific time and only for a predetermined time period. Accordingly, each TDMA user does not transmit continuously but only in its specific time slot. Therefore, for ten users within a TDMA system, each one would transmit only one-tenth of the total transmission time. Furthermore, since each user within a TDMA system is transmitting only a portion of the time, each user will be required to transmit at high rates over a shorter time interval as the number of users increase. TDMA systems also require some form of synchronization between the transmitter and its respective receiver.
Code division multiple access (CDMA) systems involve each user transmitting at the same time and at the same frequency. Further, CDMA systems perform spread spectrum techniques by multiplying the transmit sequence by a pseudo-random pattern of ones and zeros of which the receiver to receive the transmitted sequence knows. However, while CDMA systems have a “soft-capacity” in that additional users may be added with only slight system degradation, such systems are not efficient when transmitting only short bursts of data at low duty cycles.
Other systems have used CDMA and TDMA techniques to transmit bursts of data while efficiently using the frequency spectrum and supporting a large number of transmitters/users. In these systems, many users of the communications system share a common frequency band, but each transmit their respective message bursts of data at different and specific times via a TDMA technique, as defined by the system.
Unfortunately, these systems require synchronization between transmitters and receivers requiring a full duplex radio link and have been limited in the message size that can be sent to each user. For example, some systems have been limited to a message size of 120 bits. This message size is too short for some applications.
Hence, there exists a need for an improved communications system that provides efficient use of frequency spectrum while supporting a large number of transmitters sharing a common frequency band and simplex operation and supports larger message sizes, while not requiring synchronization between transmitters and receivers.